Air-cooled generator

ABSTRACT

The present invention relates to an air-cooled generator, through which cooling air flows for the removal of heat loss, wherein the cooling air sweeps over boundary surfaces ( 23 ) acting as cooling surfaces and, in so doing, absorbs heat from these boundary surfaces ( 23 ). The heat transfer is maximized with minimal cooling air consumption since the boundary surfaces ( 23 ) are provided with distributed local elevations ( 24 ) enlarging the cooling surface and the heat transfer coefficient. In particular, local elevations in the form of pyramid-shaped or truncated-pyramid-shaped bodies ( 24, 26, 29 ) are preferred.

TECHNICAL FIELD

The present invention relates to the field of rotating electrical machines. It concerns an air-cooled generator according to the preamble of claim 1.

PRIOR ART

In the case of air-cooled generators, all losses occurring in the form of heat have to be removed via the cooling medium, for example cooling air. In this case, various surfaces of the generator are used to transfer the losses to the cooling medium (the cooling air) by convection. An adaptation of the surface geometry improves this transfer in principle.

The objective of the cooling is to prevent the temperatures of the generator elements from rising during operation above an agreed temperature. If the release of the heat loss of the generator can be improved, either lower temperatures of the generator parts are to be expected or, conversely, identical temperatures of the generator parts can be achieved with a lower volume flow rate of the cooling air, which results in lower ventilation losses.

The surfaces are already optimized for specific regions, for example by cooling ribs, which enlarge the active surface. With this type of surface adaptation, the direction of incidence of the cooling medium is of great significance. Depending on design, the incident flow cannot however contact all cooling areas of the generator in an optimal manner. In a generator as is disclosed for example in document EP 740 402 A1, such cooling areas are typically located in the region of the pole windings of the rotor. FIG. 1 shows a perspective illustration of a detail of a number of poles 11 of such a generator 10, which are fastened to the rotor (not shown) by means of corresponding pole claws 13. Each of the poles 11 has a pole winding 12. The individual poles 11 are separated from one another in the circumferential direction by pole gaps 14.

The temperature calculation in air-cooled machines is based on the following physical formula:

{dot over (Q)}=α·A·ΔT

in which:

-   -   {dot over (Q)}=heat flow [W]     -   α=heat transfer coefficient [W/m2K]     -   A=cooling surface [m2]     -   ΔT=temperature difference [K]

The losses or heat loss flows {dot over (Q)}, which have to be removed, are generally known. ΔT represents the target value of the design. Since ΔT represents the temperature difference between cooling medium and cooling surface of the body to be cooled and the temperature of the cooling medium is fixed, it is possible to establish the temperature of the body to be cooled. This leaves the heat transfer coefficient and the cooling surface, which can be influenced.

The simplest cooling geometry is a smooth surface. This geometry is indeed independent of the direction of incidence, but has the minimal possible surface. In addition, the losses that can be removed per unit of area can be improved only moderately.

If a switch is made to the conventional geometries to improve the cooling, such as different embodiments of the cooling ribs (see FIGS. 4 and 5), there is the problem that the flow, due to the construction, is not always “ideal” with respect to the geometry and therefore no longer has the desired effect on the cooling. FIG. 4 shows a perspective illustration of a detail of a boundary surface 17, which has elongate ribs 18 running in parallel, which are separated from one another by recessed interspaces 19. The cross-sectional contour is rectangular and meandering. FIG. 5 shows a perspective illustration of a detail of a boundary surface 20, which has elongate ribs 21 miming in parallel, which are separated from one another by recessed interspaces 22. The cross-sectional contour is sawtooth-shaped. If these boundary surfaces 17 and 20 are used as cooling surfaces, wherein the ribs 18 and 21 act as cooling ribs, the effect thereof is highly dependent on the direction of flow of the cooling medium flowing over them.

FIGS. 6 and 7 clearly show that a misaligned incident flow of the cooling ribs 18 and 21 (direction of flow transverse to the cooling ribs) can lead to regions, specifically the recessed interspaces 19 and 22, in which the flow speeds are very low or recirculations or dead water are present, because the main portion of the flow glides away over the interspaces. In both cases this means that there is hardly any material exchange of the cooling medium in these regions. In the case of recirculations, a specific heat transfer coefficient is indeed still maintained, but both cases lead to a strong increase in the temperature of the cooling medium. Under consideration of the above-stated formula for the calculation of the heat transfer, a small temperature difference ΔT thus leads inevitably to a sharp drop in the removed heat. Conversely, this means that the surface temperature has to rise in order to remove the same amount of heat.

DISCLOSURE OF THE INVENTION

The object of the invention is therefore to specify an air-cooled generator, which avoids the described disadvantages and is characterized in particular in that a maximum heat removal from the interior of the generator is achieved with a minimal volume flow rate of cooling air.

The object is achieved by all the features in claim 1. The generator according to the invention, through which cooling air flows for the removal of heat loss, wherein the cooling air sweeps over boundary surfaces acting as cooling surfaces and, as a result, heat from these boundary surfaces is absorbed, is characterized in that the boundary surfaces are provided with distributed local elevations enlarging the cooling surface.

An embodiment of the invention is characterized in that the local elevations are distributed uniformly over the cooling surface and form a pattern.

In particular, the local elevations may have the form of simple geometric bodies.

In accordance with a development, the local elevations have the form of cones or truncated cones.

Another development is characterized in that the local elevations have the form of cylinders or rectangular parallelepipeds.

In accordance with a particularly preferred development, the local elevations have the form of pyramids or truncated pyramids.

A surface equipped with pyramid-shaped or pyramid-like elevations not only promotes the turbulence of the cooling medium flowing past, but also prevents or reduces the formation of a thermal boundary layer in the region close to the wall by deflecting flowing cooling medium away from the surface to be cooled and thus promoting mixing of the coolant perpendicular to the direction of flow.

Another embodiment of the invention is characterized in that the generator comprises a rotor with a plurality of poles, which are separated from one another by pole gaps and are each provided with a pole winding, and in that the cooling surfaces provided with the local elevations are arranged in the pole gap regions.

A further embodiment of the invention is characterized in that the generator comprises a rotor with a plurality of poles, which are separated from one another by pole gaps and are each provided with a pole winding, and in that the cooling surfaces provided with the local elevations are arranged in the region of the rear ventilation of the pole windings.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in greater detail hereinafter on the basis of exemplary embodiments in conjunction with the drawing, in which:

FIG. 1 shows a perspective view of a detail of a generator rotor with a plurality of poles, which are cooled by means of cooling air flowing through;

FIG. 2 shows the pole gaps of the rotor according to FIG. 1 for the cooling;

FIG. 3 shows the inlets for the rear ventilation of the pole windings, these inlets being important for the cooling;

FIG. 4 shows a perspective illustration of a detail from a boundary surface with parallel cooling ribs and meandering rectangular cross-sectional contour;

FIG. 5 shows a perspective illustration of a detail from a boundary surface with parallel cooling ribs and sawtooth-shaped cross-sectional contour;

FIG. 6 shows the effect of the misaligned incident flow of a boundary surface according to FIG. 4;

FIG. 7 shows the effect of the misaligned incident flow of a boundary surface according to FIG. 5;

FIG. 8 shows a perspective illustration of the detail from a boundary surface suitable as a cooling surface with local elevations in the form of pyramids with rectangular base area in accordance with an exemplary embodiment of the invention;

FIG. 9 shows various sub-figures of other types of local elevations in the form of simple geometrical bodies in accordance with other exemplary embodiments of the invention.

EMBODIMENTS OF THE INVENTION

The following aspects are taken into consideration with the cooling surface geometry forming the basis of the present invention: on the one hand a maximum heat transfer coefficient with a constant material exchange is achieved in the vicinity of the surface to be cooled. On the other hand, an enlargement of the cooling surface is achieved. In addition to these three positive points, the cooling effect is to be independent of the incident flow conditions where possible. All this is achieved since the cooling surface is provided with local elevations, which are distributed over the surface such that a uniformly high heat transfer between the cooling surface and the cooling air is achieved largely irrespectively of the direction of flow of the cooling air flowing over said cooling surface.

A perspective illustration of a detail from a boundary surface suitable as a cooling surface with local elevations in the form of pyramids 24 with rectangular (for example square) base area and corresponding interspaces in accordance with an exemplary embodiment of the invention is reproduced in FIG. 8. Here, size (base area and height) and shape of the individual pyramids 24 as well as the number and density or positioning of the individual pyramids 24 relative to one another (position of the interspaces) can be adapted within wide limits to the cooling requirements and spatial conditions in the region to be cooled in order to achieve an optimal result. Meanwhile, the pyramid shape is in no way limited to the use of pyramids or truncated pyramids having a quadrangular plan. Of course, pyramid-shaped bodies with triangular or polygonal base area can also be used. The pyramid also does not necessarily have to be a regular pyramid, that is to say not all side edges of the pyramid or of the truncated pyramid have to be of equal length.

Regions with very low flow speeds are avoided as a result of the redesigning of the surface geometry in the stated manner. Since, as can be seen from FIG. 8, cooling medium flows over practically the entire surface of the individual pyramids 24 as a result of the regular arrangement of said pyramids provided with interspaces, this leads to increased turbulences in the region of the boundary surface or cooling surface 23. This results on the one hand in a good heat transfer coefficient a and on the other hand in significantly improved mixing of the cooling medium at the surface. Points at which the cooling medium could be heated excessively intensely are thus avoided. Due to the increased heat transfer coefficient a and the enlarged cooling surface, the removed heat loss volume {dot over (Q)} also increases with constant cooling surface temperature. Conversely, the surface temperature decreases with the same losses to be removed.

A significant advantage of the pyramid structure also lies in a further effect. Compared to differently shaped local elevations, a pyramid structure as reproduced by way of example in FIG. 8 increases not only the available heat transfer area and the turbulence of the flowing cooling medium, but also promotes the material exchange between layers of the cooling medium arranged close to the wall and far from the wall, that is to say contributes to a mixing of said layers perpendicular to the direction of flow. Since the cooling medium flows against the side faces of the pyramid-shaped bodies (24, 26, 29), it is deflected away from the wall to be cooled. A flow component directed away from the surface to be cooled is formed, in the wake of which fresh cool medium is led toward the wall. The formation of a thermal boundary layer is thus impaired or avoided. In contrast to ribs subject to a lateral incident flow (see FIG. 7), no dead water is formed with the pyramid-shaped bodies according to the invention. In order to fully utilize this advantage, triangular or quadrangular pyramid-shaped elevations are preferable in accordance with an advantageous embodiment and are arranged such that a side face is oriented perpendicular to the primary direction of flow.

The prior embodiments are based on local elevations in the form of a pyramid 24. It is quite possible however within the scope of the invention to use other geometries. Here, a cone 25 (FIG. 9 a) or truncated cone is shown by way of example in accordance with FIG. 9. Pyramids 26 with triangular base area as well as truncated pyramids 29 with a flattened tip are illustrated in FIGS. 9 b and 9 e.

On the other hand, cuboids, cylinders 27 (FIG. 9 c), rectangular parallelepipeds 28 (FIG. 9 d) and further prisms could advantageously be considered as basic elements for the cooling surface geometry, wherein, in these cases, the previously explained additional advantages of a pyramid-shaped surface structure, specifically the reduced formation of a thermal boundary layer, are not provided however.

Preferred fields of application of the present invention in accordance with FIG. 2 are, above all, the pole gap regions 15 of the generator 10, for example at one or more pole winding surfaces or pole body surfaces, and in accordance with FIG. 3 the rear ventilation of the pole windings 12. 

1. An air-cooled generator (10), through which cooling air flows for the removal of heat loss, wherein the cooling air sweeps over boundary surfaces (23) acting as cooling surfaces and, in so doing, absorbs heat from these boundary surfaces (23), characterized in that the boundary surfaces (23) are provided with distributed local elevations (24-29) enlarging the cooling surface.
 2. The generator as claimed in claim 1, wherein the local elevations (24-29) are distributed uniformly over the cooling surface (23) and form a pattern.
 3. The generator as claimed in claim 1, wherein the local elevations have the form of simple geometric bodies.
 4. The generator as claimed in claim 3, wherein the local elevations have the form of pyramids (24, 26) or truncated pyramids (29).
 5. The generator as claimed in claim 4, wherein the pyramids (24, 26) or truncated pyramids (29) have a quadrangular base area.
 6. The generator as claimed in claim 5, wherein the pyramids (24, 26) or truncated pyramids (29) have a rectangular base area.
 7. The generator as claimed in claim 4, wherein the pyramids (24, 26) or truncated pyramids (29) have a triangular base area and in particular are formed as tetrahedrons.
 8. The generator as claimed in claim 4, wherein a side face of the pyramids (24, 26) or of the truncated pyramid (29) subject to an incident flow is oriented transverse to the primary direction of flow of the cooling medium.
 9. The generator as claimed in claim 6, wherein a longer side edge of the pyramids (24, 26) or of the truncated pyramid (29) is oriented transverse to the primary direction of flow of the cooling medium.
 10. The generator as claimed in claim 3, wherein the local elevations have the form of cones (25) or truncated cones.
 11. The generator as claimed in claim 3, wherein the local elevations have the form of cylinders (27) or rectangular parallelepipeds (28).
 12. The generator as claimed in claim 1, wherein the generator (10) comprises a rotor with a plurality of poles (11), which are separated from one another by pole gaps (14) and are each provided with a pole winding (12), and in that the cooling surfaces provided with the local elevations (24-29) are arranged in the pole gap region (15), for example on one or more pole winding surfaces or pole body surfaces.
 13. The generator as claimed in claim 1, wherein the generator (10) comprises a rotor with a polarity of poles (11), which are separated from one another by pole gaps (14) and are each provided with a pole winding (12), and in that the cooling surfaces provided with the local elevations (24-29) are arranged in the region of the rear ventilation (16) of the pole windings (12).
 14. The generator as claimed in claim 2, wherein the local elevations have the form of simple geometric bodies.
 15. The generator as claimed in claim 6, wherein a side face of the pyramids (24, 26) or of the truncated pyramid (29) subject to an incident flow is oriented transverse to the primary direction of flow of the cooling medium and wherein a longer side edge of the pyramids (24, 26) or of the truncated pyramid (29) is oriented transverse to the primary direction of flow of the cooling medium. 